Spectral sensitization techniques are very important, indeed indispensable, in the manufacture of photosensitive materials which have high photographic speeds and excellent color reproduction characteristics. Spectral sensitizing agents have the effect of absorbing long wavelength light which is essentially unabsorbed by silver halide photographic emulsions in practice and transmitting the energy of the light which has been absorbed to the silver halide. Hence, increasing the amount of light captured by the spectral sensitizing agents is useful for increasing photographic speed. Consequently, attempts have been made to increase the amount of light which is captured by increasing the amount of spectral sensitizing agent which is added to the silver halide emulsion. However, if more than an optimum amount of a spectral sensitizing agent is added to a silver halide emulsion it results in a pronounced loss of photographic speed. This is known generally as dye desensitization, and it is a phenomena in which desensitization occurs in the photosensitive region which the silver halide intrinsically processes, wherein essentially no light is absorbed by the sensitizing dye. In those cases where dye desensitization occurs to a substantial degree, there is inevitably an overall loss of photographic speed even though a spectral sensitizing effect is present. In other words, if dye desensitization is reduced, the speed in the region in which light is absorbed (which is to say the spectrally sensitized speed) due to the sensitizing dye is increased by the same amount. Hence, improvement in respect of dye desensitization is a major theme in spectral sensitization technology. Furthermore, in general terms, dye desensitization increases as the photosensitive region of the sensitizing dyes shifts to longer wavelengths. This fact has been described by C. E. K. Mees in The Theory of the Photographic Process, pages 1067-1069 (Published by Macmillan, 1942).
Known methods of reducing dye desensitization and increasing photographic speed have been disclosed in JP-A No. 47-28916, JP-A No. 49-46738, JP-A No. 54-118236 and U.S. Pat. No. 4,011,083. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".) However, the sensitizing dyes which can be used in the aforementioned techniques are limited and the effect obtained is still unsatisfactory. At the present time, the most effective known means for the amelioration of dye desensitization involves the conjoint use of the pyrimidine derivatives or triazine derivatives disclosed, for example, in JP-B No. 45-22189, JP-A No. 54-18726, JP-A No. 52-4822, JP-A No. 52-151026 and U.S. Pat. No. 2,945,762, and substituted bis-aminostilbene compounds. (The term "JP-B" as used herein signifies an "examined Japanese patent publication".) However, the aforementioned compounds are effective only in the case of so-called M-band sensitizing type dyes which have a gently sloping sensitizing peak, such as dicarbocyanine dyes, tricarbocyanine dyes, rhodacyanine dyes and merocyanine dyes for example, and dyes which have a sensitizing peak at a comparatively long wavelength.
The fact that sensitization can be achieved in the infrared region with combinations of specified tricarbocyanine dyes and ascorbic acid has been disclosed in U.S. Pat. No. 3,695,888, the fact that the minus blue speed can be raised by the conjoint use of specified complex merocyanine dyes and ascorbic acid has been disclosed in British Patent No. 1,255,084, the fact that an increase in speed can be attained with the conjoint use of specified complex cyanine dyes and ascorbic acid has been disclosed in British Patent No. 1,064,193, and the conjoint use of super-sensitizing agents such as ascorbic acid with desensitizing nuclei containing cyanine dyes has been disclosed in U.S. Pat. No. 3,809,561.
However, in none of the above mentioned conventional techniques is the sensitizing effect of the dye really satisfactory.